APPLICATION NOTE

APPLICATION NOTE
Storing Power with Super Capacitors
Storing Power with Super Capacitors
Portable system designers have sharply reduced the power requirements of their systems in active and standby mode
over the last several years, a growing number of applications pose an entirely different problem. Some of the most
attractive features in wireless and portable devices today demand high levels of current for very short durations. Often,
this peak current exceeds the capabilities of the system power source. Ironically, as advances in power management
have allowed designers to reduce the footprint and cost of their systems by moving to smaller, lower cost Lithium-Ion
batteries, they are finding users increasingly demanding applications that require higher levels of peak current than
these new power sources can support.
This problem has emerged as designers have increasingly deployed higher current devices in portable systems despite
their limited sourcing current. For example, a growing variety of wireless data cards for applications such as WCDMAHSPA and GSM/EDGE networks, GPRS and WiMAX data communication use TDMA and CDMA techniques which require
a peak current during the transmission of signals which can exceed the maximum current specified in the PC card, CF
Card and/or USB standards. Similarly, as designers have increased the resolution of camera phones to 3Mpixel and
beyond, they have also increased the amount of light required to achieve a high quality image. To reach these high
levels, portable systems must drive flash LEDs at currents as high as 4A. Other applications such as GPS readings,
music and video also exceed source current availability.
One way to solve this problem is to use a capacitor to store the current and deliver it quickly without draining the main
battery. However, conventional capacitor capability would require either a very large case size or multiple devices connected in parallel. A more practical solution for space-constrained portable systems is to use very high value or so-called
“super” capacitors. These devices offer high levels of capacitance in a relatively small case size. Working with the battery and a DC/DC converter, a super capacitor discharges its power during peak loads and recharges between peaks,
providing the power needed to operate systems from battery operated hosts.
By using a super capacitor (SC), designers can deliver the high current levels needed for these short duration events
and then recharge from the battery between events. By reducing the current drawn from the battery, this architecture
offers users improved talk time and longer battery life. It also allows the designer to reduce the system footprint by
optimally sizing the battery and power circuitry to cover just the average power consumption instead of peak levels.
The challenge for designers is determining how to most efficiently interconnect the battery, DC/DC converter and SC
in a way that will limit the super capacitor charge current and continually recharge the capacitor between load events.
This app note will identify some of the challenges of storing power with super capacitors, identify some of the issues
designers must overcome, explore two techniques for delivering high currents without overloading the host supply, and
then illustrate potential solutions using implementations of sample applications with test results from a real system.
Defining a Super Capacitor
What is a super capacitor? Like any capacitor, a super capacitor is basically two parallel conducting plates separated by
an insulating material known as a dielectric. The value of the capacitor is directly proportional to the area of the plates
and inversely proportional to the thickness of the dielectric. Manufacturers building “super” capacitors achieve higher
levels of capacitance while minimizing size by using a porous carbon material for the plates to maximize the surface
area and a molecularly thin electrolyte as the dielectric to minimize the distance between the plates. Using this approach
they can manufacture capacitors with values from 16mF up to 2.3F. The construction of these devices results in a very
low internal resistance allowing them to deliver high peak current pulses with minimal droop in the output voltage.
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1
APPLICATION NOTE
Storing Power with Super Capacitors
Manufacturer
Part Number
Capacitance (F)
Voltage Rating (V)
ESR
(mW)
Size (mm)
LxWxH
TDK
CAP-XX
EDLC262020-501-2F50
HA230
0.5
0.425
5.6
5.5
50
110
23x44x1.5
20x18x1.5
Benefits
High-Capacity Capacitor
Farads rather than micro-Farads
Values ranging from 30mF to 2F
Recharge in seconds with >500k Cycles
Stores energy in an electrostatic field, not a chemical state
Small ESR, low impedance
30 – 185mΩ
Extends battery life by five times
‘Averages out' high power demands
Manufactured in any size and shape
Flat and small sizes
Allows smaller, lighter and cheaper batteries.
Long life: 10 to 12 yrs
Open-circuit (high ESR) failure mode
Carbon, aluminum, organic electrolyte
Challenges
Need to control inrush charge current due to low ESR
Need to recharge
Voltage drop/droop is below operational limit of system requirement
Need to disconnect SC from source
Short circuit protection
Source over-voltage protection
Current flow protection
Maximum voltage, temperature range, ESR
2.75/cell to 5.5V, 70°C to 85°C, 30 to 185mΩ
Need cell balancing resistors required for stacked SC for higher operating voltage
Figure 1: Example Portable Super Capacitors; Size Continues to Fall as Voltage and Values Rise.
There are both benefits and challenges to overcome when designing with super capacitors.
2
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APPLICATION NOTE
Storing Power with Super Capacitors
Super Capacitor Benefits
Conventional capacitor technology requires either a very large case size or multiple devices connected in parallel to
achieve high capacitance values. Super capacitors can be manufactured in a smaller size for a given capacitance and
shape. A super capacitor can be used to store the required current and deliver it quickly without draining the main
battery. Working together with the battery, the super capacitor discharges its power during peak loads and recharges
between peaks, providing the power needed to operate systems from battery-operated hosts up to 200% longer while
extending the life of the battery. They can be used to extend battery life by five times by ‘averaging out' high power
demands and therefore allow designers to use smaller, lighter and cheaper batteries. Super capacitors also offer an
operating life as long as 10 to 12 years and their failure mode is an open-circuit (high ESR), rather than a battery’s
destructive event. Similarly, if over-voltage is applied to the device, the only consequence will be a slight swelling and
a rise in ESR, eventually progressing to an open circuit. Super capacitors recharge in seconds with >500k cycles and
store energy in an electrostatic field rather than a chemical state like a battery. Their small ESR and low impedance
also helps ensure that the voltage does not droop excessively until heavy load currents when fully charged.
Super Capacitor Challenges
The challenge for designers is how to efficiently interconnect the battery, DC/DC converter and super capacitor in a
way that will limit the super capacitor inrush charge current and continually recharge the capacitor between load
events. The low ESR presents designers with an inherent problem during the initial charge cycle. In any system the
capacitor is initially discharged; when the supply voltage is then applied, the super capacitor looks like a low value
resistor. This results in a huge inrush current if the current is not controlled or limited. Therefore, designers must implement some sort of inrush current limit to ensure the battery does not shut down. In addition, the super capacitor must
be recharged when the voltage drops/droops below the operational limit of the load. When the super capacitor is fully
charged, it must then be disconnected from the source. Typically any circuit of this type also requires short-circuit,
over-voltage and current flow protection.
One simple strategy is to use a series resistor. In a typical PC card circuit the maximum current that can be drawn prior
to successful host/card negotiation is 70mA. If we assume that the PC card controller needs half that current to perform
the negotiation, then at power-up the super capacitor must be either disconnected from the supply or current-limited
using an approximately 100Ω resistor (R = V/I). Given those factors, the capacitor will be fully charged in approximately 6.7 minutes (assuming the capacitor is fully charged in approximately 5 time constants).
A more practical approach would allow the PC card to source more power after the successful negotiation between the
host and the card. A lower value resistor can then be used to increase the charging current. As the capacitor starts to
charge and the voltage starts to rise, the power dissipation declines and the resistor value can be decreased.
Figure 2 depicts a sample circuit comprised of a series of decreasing resistor values which are switched during the
capacitor charging cycle. This architecture requires that the timing of the switching points be closely controlled, which
demands very accurate and expensive resistors, or monitored by several additional voltage detectors. Furthermore, when
the capacitor is fully charged and the PC card is removed, the energy stored in the capacitor would be sufficient to damage the connector pin. Such a scheme is inexpensive, but a control to switch in different accurate R values requires timing and monitoring. There is also a loss across the resistor and a very long charge time if a single R is used.
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APPLICATION NOTE
Storing Power with Super Capacitors
DC-DC
VCC From PC Card
3
2
1
LOAD
Sequence:
1 - On card plug in
2 – Intermediate Limit
3 – Final Limit
GND From PC Card
SuperCap
Figure 2: Simple Charging Scheme.
A third approach is to use a current-limiting SmartSwitch™ to charge the super capacitor. This type of device uses an
independent integrated P- or N-channel MOSFET as a load switch and integrates additional monitoring and protection
circuitry to limit the amount of output current. Products of this type feature thermal overload protection to ensure that
the device turns off if the chip temperature exceeds its rated maximum while in current limit. As the chip cools down,
the device will turn back on and thermally oscillate at a low frequency until the period of high dissipation ends. These
devices typically feature an independent current limit to avoid violating standards or draining the battery. They also
feature reverse blocking to keep the super capacitor charge from going back to the source and to protect the source
during a short circuit. They offer a power loop to control the charge rate and low RDS(ON) to eliminate thermal foldback
or droop in the output voltage. Moreover, these devices only require 1.4V for the enable control pin to set startup and
full power current.
A current-limited SmartSwitch™ adds all the circuitry needed to limit current, protect the PC card connector, continuously charge the capacitor, notify the system when it is ready for use, and determine when to start recharging the
capacitor.
4
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APPLICATION NOTE
Storing Power with Super Capacitors
VCC
100kΩ
Reverse
Blocking
VCC
Over-Temp
Protection
UnderVoltage
Lockout
Power
Loop
OUT
To Load
RDY
To μC
EN IL
Load
EN IU
1.2V
Reference
ISETL
Current
Limit
Control
ISETU
GND
RHYS
RSETU
RSETL
SuperCap
Figure 3: Current Limiting SmartSwitch™ with Independent Current Limits from 75mA
to 1200mA and Soft-Start Used to Overcome Initial Charging of the Super Capacitor.
A power loop (thermally activated current reduction) minimizes charge time while controlling power
dissipation with over-temperature and short circuit protection. Reverse current blocking prevents
discharge of the super capacitor back to the power supply and 50-65mΩ typical RDS(ON) charges the
capacitor voltage close to the input supply. A system READY output alerts the system that the super
capacitor is charged and ready with programmable hysteresis for adjustable recharge.
In a typical super capacitor application, the SmartSwitch™ turns on and immediately limits current due to the high
in-rush current. This large in-rush current drives up temperature and drives the device into thermal shutdown where
it thermally oscillates. All SmartSwitch™ devices are designed to operate in this manner. However, during the time the
switch is off, the capacitor is not charging, and therefore increasing the time to full charge. Moreover, there is no way
to detect when the capacitor is fully charged and ready for transmission without additional circuitry.
Devices such as Skyworks SmartSwitch™ feature two different current limits for host/card negotiation. On power up
the device provides a low resistance path between the supply and the super capacitor. If thermal dissipation is low, the
device eventually enters current limit and the capacitor continues to charge until it reaches approximately 98% of its
final value. At that point a system ready signal changes state alerting the system that transmission can begin. If thermal dissipation is high, the chip temperature will rise rapidly. When it reaches an internally programmed limit, the
device initiates an integrated digital power loop which reduces the current to a safe value. The power loop regulates
the die’s temperature to approximately 100°C by sensing the die temperature at regular intervals and increasing or
decreasing the current by 1/32 of the current limit set point. This function protects the device while minimizing charge
time by ensuring that the super capacitor is charging at all times. Typically designers size the super capacitor to
minimize the voltage droop during transmission and allow recharging during the receive phase. The SmartSwitch™
adds adjustable hysteresis for a Ready (RDYB) signal. The RHYS resistor sets the hysteresis in which the RDYB signal is
turned back off. The RDYB signal always turns on when VOUT = 0.98 · VIN. The SmartSwitch™ stops charging the supercap when VOUT > (VIN - 18mV). It then turns back on when VOUT < (VIN - 18mV-hysteresis).
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APPLICATION NOTE
Storing Power with Super Capacitors
GSM/GPRS Application Example
A GSM/GPRS wireless data card design in a current limited environment is used to demonstrate the advantages of this
approach. GPRS type data cards typically must supply relatively large amounts of current to the power amplifier (PA) to
send data for short durations. However, successive data transmissions can extend the time needed to supply the PA. The
reason that the super capacitor is useful is because the data card is plugged into either a limited supply current PC or
CF card or USB slot on a laptop. The supply currents for three different portable standards are listed in Table 1.
GPRS (General Packet Radio Service) is defined in classes as shown in Table 2. The first column is the GPRS multislot
class, which is a list of speeds. The second column is the downlink timeslots receiving data from the network. The third
column is the uplink timeslots to transmit data to the network. The most common GPRS multislot classes are Class 8,
which has 33% faster data download than Classes 4 and 6, and Class 10 which has better data uploading than Class 8
and is used in cell phones and PC/Express cards. Class 12 has maximum upload performance for high-end PC cards.
In the example shown in Figures 4, 5, and 6, when the GPRS card uses one slot @ +33 dBm, the current draw from
the super capacitor is 1.1A for 577µs/4.615ms or a 12.5% duty cycle. PA power consumption is equivalent to +33 dBm
(50% efficiency) or 2W/50% or 4W. Current during transmit was 4W/3.75V or 1.1A. Average current was 1.1A · 12.5%
of the duty cycle or 138mA. A second 2-slot example drew 1.1A from the super capacitor for 1.154 ms every 4.615ms.
At 2 slots per frame this represented 25% of the duty cycle. PA power consumption and current during transmit was
the same but average current was 1.1A · 25% or 275mA. The actual measured PA current was approximately 2A during the test.
PC Card
Voltage
Current Level 0 (max)
Current Level 1 (max)
3.3V ± 10%
5.0V ± 10%
70mA
100mA
1000mA
1000mA
CF Card
Voltage
Current Level 0 (max)
Current Level 1 (max)
3.3V ± 5%
5.0V ± 10%
75mA
100mA
500mA
500mA
USB Port
Voltage
Current Level 0 (max)
Current Level 1 (max)
5.0V ± 10%
100mA
500mA
Table 1: Maximum Supply Currents For Laptop Standard Ports.
Multislot Class
Downlink Slots
Uplink Slots
2
4
6
8
10
12
32
2
3
3
4
4
4
5
1
1
2
1
2
4
3
Table 2: GPRS Multislot Classes.
6
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APPLICATION NOTE
Storing Power with Super Capacitors
GPRS Transmit = PA drive current
Class 8 (4R1T) = 4 receive slots and 1 transmit timeslot
Class 10 (3R2T) = 3 receive slots and 2 transmit timeslots
GSM FRAME = 8 TIMESLOTS
577µs per Slot/ 4.615mS Total
CLASS 8
GPRS Transmit 1-Slot Ch975, 33dBm
CLASS 10
GPRS Transmit 2-Slot Ch975, 33dBm
VIN = 3.83V
4
2.5
VOU T = 3.80V
2
3.0
2.0
1
1.5
Transmit
1-Slot
0
-1
Receive
4-Slots
IOUT
1.0
0.5
3.5
VIN = 3.83 V
3
3.0
2.5
VOUT = 3.76 V
2
2.0
1
Receive
4-Slots
Transmit
2-Slots
0
-1
-2
0.0
-2
-3
-0.5
-3
IOUT
1.5
1.0
0.5
Output Current (A) Bot
3
5
Output Current (A) Bot
Input Voltage (V) Top
Output Voltage (V) Mid
4
3.5
Input Voltage (V) Top
Output Voltage (V) Mid
5
0.0
-0.5
Time (1ms/div)
Time (1ms/div)
Figure 4: GSM, GPRS Data Communications Slots and Transmit/Receive
Functions and Test Results for Class 8 and Class 10.
3.7V System Power
Battery
Inp ut:
3.3V
Boost DC/DC
VCC
L=4.7u H
O UT
LX
EN H
ENL
ISET U
FB
RSET
CT
AG ND
UltraCap
PGN D
Startup waveform no load 96mA current limit
CH4 VCC
VCC
VOUT tr=2.5ms
/RDY
CH 1 /RDY
CH 2 II N
ISET L
Cout
Startup waveform no load 800mA current limit
CH 3 V OUT
Power Amp
Control
E N/SET
CH 4 V CC
/RDY
Current Limiting Smart
Switch
IIN = 800mA
CH 4: VCC, 1V/div
CH 3: VOUT, 1V/ div
CH 2: IIN, 5 00mA/div
CH 1: /RDY, 1V/div
T IME: 500 ms/div
CH3 V OUT
CH1 /RDY
CH2 IIN
VCC
VO UT tr=20s
/RDY
C H4: V CC, 1V/div
C H3: V OUT, 1V/div
C H2: IIN , 500mA/div
C H1: /RDY, 1V/div
TIME: 5s/div
II N = 100mA
Figure 5: 3.3V Express Card with VOUT = 3.7V, Super Capacitor = Cap-XX GW201G, 0.3F, 85mΩ.
Important features such as charge ramp of the super capacitor, RDS(ON) IR drop, the duration of the current
pulse test results. Inrush current for the super capacitor can be limited to under 100mA or up to 800mA.
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APPLICATION NOTE
Storing Power with Super Capacitors
Wireless GSM Card w/o Ultra Cap
VOUT 3.7V
VOUT 1.7V Droop
Wireless GSM Card with Ultra Cap
VOUT 3.7V
VOUT 174mV Drop
VIN 3.3V
IIN 1A
IIN 500mA
IUCOUT 2A
Figure 6: Wireless Data Card Application.
The initial IR voltage drop caused by the super capacitor ESR plus the capacitor voltage drop which is VDROP =
I x ESR+I x (tms/CmF) = 2A x 85mW + (2 x (1.154ms/300mF) = 174mV.
In this demonstration, the battery supplies input to the boost converter which supplies a constant system supply of
3.7V. The super capacitor supplies approximately 1.5A of peak current during GPRS 2-slot data transmission and the
SmartSwitch™ recharges the super capacitor during receive function. The two waveforms in Figure 6 compare a GSM
PC card without a super capacitor load (left) with a GSM PC card with a super capacitor load (right). Together they
illustrate how designs using a SmartSwitch™ to charge the super capacitor can eliminate droop during transmit. Super
capacitors can operate high current devices such as rf PAs during transmit for load leveling, and energy storage
between transmits. Super capacitors are getting smaller with improved specs such as temperature range (85°C) and
maximum voltage limits (5.5V). A current limiting SmartSwitch™ alleviates the issues with super capacitor inrush current and provides short-circuit protection, source OV protection, and current flow protection to meet portable standards
and run time requirements without taxing the battery.
Flash LED Application Example
In a second example, a current limiting SmartSwitch™ with a boost DC/DC LED driver and super capacitor is used in a
high current flash LED application. The SmartSwitch™ is integrated with the boost DC/DC LED driver as shown. Many of
today’s camera phones now offer resolution of 3MP (megapixel) and above, as shown in Figure 7. These higher levels of
image resolution require proportionally higher levels of light to achieve a high quality image. Image sensors need more
light for high MP photos and first generation camera phone flash offer limited light intensity. Second generation camera
phone flash is still not suitable for >3MP cameras and they also require video capture which needs a movie/torch
mode.
8
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APPLICATION NOTE
Storing Power with Super Capacitors
Source: 2005-2010 Strategies Analytics
Figure 7: The Camera Phone Market Continues to Grow.
To achieve these light levels, flash LEDs must be driven at currents of between 1A and 2A, as shown in Figure 8.
Complicating the problem, the forward voltage across the LED as these high currents can range up to 4.8V. If we
include 200 mV of overhead for the current control circuitry, the total load voltage during a flash event can range up
to 5V.
Measured PWM-4 IF vs VF
Forward Current (mA)
3000
VF Peak
VF Cont.
2500
Req. HR
SCap Voltage
2000
1500
1000
500
0
3
3.5
4
4.5
5
5.5
6
Forward Voltage (Volts)
Figure 8: High Intensity Flash LEDs Require Large Forward Current (IF) and High Forward Voltage (VF).
Since the forward voltage of the flash LED is proportional to IF, flash LEDs require >1A. To achieve 2A, the VF must be
>4.8V.
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APPLICATION NOTE
Storing Power with Super Capacitors
Current solutions consist of a WLED flash which is charge-pump based and limited to up to 800mA, or an inductive based
step-up converter solution which is limited to 1-1.5A due to high input battery current. If the battery voltage is 3.5V, and
the boost converter is 90% efficient, the battery would need to supply over 3A for the duration of a 2A flash pulse.
If VBAT = 3.3V and IF = 1.5A @ VF = 4.4V, then:
IBAT =
=
VF · IF
Eff · VBAT
4.4 · 1.5
0.85 · 3.3
= 2.35A
This current can cause a number of issues. Peak current can create a battery voltage drop, causing the camera or phone
to reset. If the current exceeds the maximum allowed battery current, the battery disconnects for safety; this will either
cause the battery protection circuit to shut the battery down or cause a low voltage shutdown with plenty of energy
still remaining in the battery or the battery is damaged.
Another available solution is Xenon flash, but it requires high voltage, 300V with ignition at several kVs. It is bulky and
needs a reflector and high voltage capacitor. Also, Xenon flash cannot operate in torch mode and is very fragile, as
shown in Figure 9.
Figure 9: Xenon vs. Super Capacitor Flash.
The super capacitor can provide LED flash better than Xenon flash in a solution which is a fraction of the size,
and provide torch mode with true DSC performance.
10
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APPLICATION NOTE
Storing Power with Super Capacitors
A super capacitor-powered LED flash unit can drive high-current LEDs to provide light intensity many times greater
than standard battery-powered LED flash units or longer than Xenon strobe solutions. The super capacitor solution
shown in Figure 10 contains a step-up converter used to boost the 3.2V-to-4.2V battery input voltage up to a constant
5.5V and a SmartSwitch™ to charge a super capacitor. The super capacitor holds the charge for the flash LEDs until
needed by the user in either flash or movie mode. The solution controls and regulates the current from a cell phone’s
battery source, steps up the battery voltage, and manages the charging of a super cap for the control and supply of
high-current to flash LEDs in the end application. This solution also contains flash management capabilities such as
movie-mode and red eye reduction.
SW
AAT128x
IN
OUT
SuperCap
Step-up/Boost
Converter with
Smart Switch
3.2-4.2V
FLEN
SDA
SCL
Digital interface
FLA
EN
FLB
Figure 10: Flash LED Operation with a Super Capacitor.
1. After enable (EN), step-up converter charges the super capacitor to 5.5V in seconds (Red).
2. The step-up converter automatically changes to light load mode.
3. During the flash (FLEN), the boost engine is shut down and the two LEDs connected to current channel
(FLA or FLB) share the output current supplied by the super capacitor equally (Blue).
To better achieve this, the step-up converter features built-in circuitry that prevents excessive inrush current during
start-up as well as a fixed input current limiter of 800mA or 500mA for USB and true load disconnect after the super
capacitor is charged. The output voltage is limited by internal overvoltage protection circuitry, which prevents damage
to the controller and super capacitor from open LED (open circuit) conditions. During an open circuit, the output voltage rises and reaches 5.5V (typical), and the OVP circuit disables the switching, preventing the output voltage from
rising higher. Once the open circuit condition is removed, switching will resume. At this point the controller will return
to normal operation and maintain an average output voltage. An industry-standard I2C serial digital input is used to
enable and disable LEDs, and set the movie-mode current with up to 16 movie-mode settings for lower light output.
The detailed schematic in Figure 11 illustrates the minimal components needed to implement this flash lighting subsystem. A 0.55F, 85mΩ super capacitor delivers 9W LED power-bursts using the AAT1282 high power 2A flash LED driver
which has the super capacitor charger integrated with the boost DC/DC LED driver. To achieve high light levels, the flash
LEDs are driven at currents of between 1A and 2A. The forward voltage (VF) across the LED at these high currents can
range up to 4.8V. If we include 200 mV of overhead for the current control circuitry, it’s easy to see how the total load
voltage during a flash event can range up to 5V and require a 5.5V step-up voltage. To achieve these levels of power,
designers must use a boost converter. If the battery voltage is 3.5V and the boost converter is 90% efficient, the battery
must supply over 3A for the duration of a 2A flash pulse. Typically this requirement will result in the battery protection
circuit shutting the battery down or at the very least create a low voltage shutdown with significant amounts of energy
left in the battery. This solution combines high-frequency boost converter with fixed input current limiting smart switch
and high-capacity super capacitor. Dual output regulated current sinks and I2C control supplies high intensity light
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11
APPLICATION NOTE
Storing Power with Super Capacitors
required for mobile phones using cameras with resolutions of > 3 MP and limits peak current consumed by the battery.
Future integrated solutions include the SmartSwitch™ with PMUs to supply portable solid state disk drives.
Smart Switch plus boost converter – super capacitor provides the flash current
Super
Capacitor
2 x WLED Flash
With Lens
Smart Switch integrated with the boost converter
Component
Solution
Figure 11: Flash LED Application.
The super capacitor is mounted on the back side of the PCB.
12
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APPLICATION NOTE
Storing Power with Super Capacitors
Figure 12 shows test results using two flash LEDs at 1A each and 1 LED at 2A. As can be seen, the super capacitor can
easily supply the necessary current for 100ms while holding up the supply voltage sufficiently above the VF of the LED.
Between flash events, the super capacitor is recharged at a slower rate to be ready for the next picture. The time to
charge the super capacitor between flashes is set externally and can be optimized for different battery sizes/chemistries.
Super Cap = Vout= 5.5V
VOUT
(500mV/div)
Super Cap Voltage
IFL1
(500mA/div)
LED2 Current
IFL2
(500mA/div)
130ms
VOUT
(500mV/div)
I FLX
(500mA/div)
LED1 Current
120ms
LED1 Current
• Vin = 3.62V (875mA/hr Li-Ion Polymer battery cell)
• Flash Time-out set for 120 to 130ms
• LEDs = Lumiled PWM-4
• Super Capacitor = Cap-XX HS206F - 055F – 85mΩ ESR
Figure 12: Flash LED Application Test Results for High Current Flash LED Application
(Two LEDs Powered at 1A Each and One LED Powered at 2A).
Since the super capacitor is the only source for the LED flash current, the duration of the flash is determined by the
energy stored in the super capacitor. During flash, the energy of the super capacitor is discharged; at the same time,
the voltage of the super capacitor is decreased. Once the super capacitor voltage is lowered to a level (the minimum
sink pin voltage + the LED forward voltage), the flash is ended. With a fully charged super capacitor in place, the flash
for two 1A LEDs can last for more than 500ms.
To determine the super capacitor size for the flash LED application, the total voltage drop has to be considered. The
initial IR voltage drop caused by the super capacitor ESR plus the voltage drop due to time that the LEDs are conducting current.
VDROP = I · ESR + I ·
•
•
•
•
•
t (ms)
C (mF)
VOUT drop to the LED VF Limits Flash time
IR drop due to SC ESR = VOUT - VMINSTEP = (5.5 - 5.4) = 100mV
ESR = 100mV/2A = 50mΩ
Voltage drop on SC = (VMINSTEP - VMINDROP) = (5.4 - 4.5) = 900mV
SCMIN = 2A(150ms)/900mv = 0.33F
For example using a super capacitor with an ESR of 50mΩ and 2A of LED forward current:
VDROP = 2A · 50mΩ + I ·
150ms
333mF
= 1V
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APPLICATION NOTE
Storing Power with Super Capacitors
The maximum flash current in each FLOUTA and FLOUTB is set by the RSET resistor and can be calculated using the following equation:
IFLOUTA = IFLOUTB =
81kΩ · A
81kΩ · A
=
= ~1000mA per channel
RSET
80.6kΩ
To prevent excessive power dissipation during higher flash current operation, RSET values smaller than 80.6kΩ are not
recommended. An example showing flash time longer than 500ms where the super capacitor voltage droops below the
VF required by the flash LEDs is shown in Figures 13 and 14.
VIN = 3.6V
Super Cap = VOUT = 5.5V
Linear
Mode
VOUT
(2V/div)
ILX
(500mA/div)
IFLx
(500mA/div)
Boost
Mode
LED Current starts to
go out of regulation
500ms
Figure 13: 1A/LED Flash Operation Test Results – Long Duration Flash.
VSW
(10V/div)
IFLOUTA
(500mA/div)
IFLOUTB
(500mA/div)
90ms
230ms
90ms
Figure 14: 1A Double Flash Operation Showing Recharge of the Super Capacitor and Turn-On/Off of the
Boost Converter Between Flashes as it is Disconnected from the Battery During the Flash Event.
14
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APPLICATION NOTE
Storing Power with Super Capacitors
Movie-Mode
Movie or torch (flashlight) mode is when the LEDs are turned on at a lower light level to record a video or movie. The
maximum movie-mode current level is set by the maximum, programmed flash current reduced by the programmed
flash-to-movie-mode ratio in which the default value is 7.3:
IMOVIE-MODE(A/B) =
IFLOUT(A/B)(MAX) 1000mA
=
= 137mA
7.3
7.3
To change the configuration or the settings, the AAT1282 can be programmed via the I2C interface. An example of
movie mode between flashes is shown in Figure 15. Only LEDs can provide movie mode; a xenon tube cannot be used
in this mode.
ENABLE
5.5V
VOUT = SuperCap voltage
I2C Control for Movie
Mode Enable
FLASH ENABLE
tCT
LED CURRENT
Turn on LEDs in
Movie Mode
controlled by I2C
interface
Flash times out
Figure 15: Double Flash Operation (Super Capacitor Recharges Between Flashes).
This solution includes a timer circuit that enables the flash current for a programmed period of time. This feature
eliminates the need for an external, housekeeping baseband controller to contain a safety delay routine. It also serves
as a protection feature to minimize thermal issues with the flash LEDs in the event an external controller’s flash software routine experiences hang-up or freeze. The flash safety timeout, T can be calculated by the following equation:
T = 13.5s/μF • CT
Where T is in seconds and CT is the capacitance of the timer capacitor in μF.
For example, using a 74nF capacitor for CT sets the flash timeout to:
Flash Safety Timeout = 13.5s/μF • 0.074μF = 1s
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APPLICATION NOTE
Storing Power with Super Capacitors
Mode 1
Mode 2
Mode 3
Time Duration
LED Current
Time Duration
LED Current
Time Duration
LED Current
2 sec
140ms
100ms
140ms
50mA
1A
OFF
1A
2 sec
70ms
100ms
70ms
50mA
1.5A
OFF
1.5A
2 sec
40ms
100ms
40ms
50mA
2A
OFF
2A
Figure 16: Flash Reference Design with 3 Modes of Operation
Depending on the Light Intensity Required for the Flash.
Xenon
Super Capacitor
Xe filled quartz tube 15x5x3mm = 0.225cc
Si LED 2x 1.6x0.7mm = 0.00448cc
Electrolytic capacitor 2x 70x18mm = 1.766cc eff
Super capacitor 1x or 2x 17x28.5x1mm = 0.48 - 0.96cc
10x30x0.2mm = 0.06cc
10x30x0.2mm = 0.06cc
Control Chip
4x4x1 = 0.016cc
4x4x1 = 0.016cc
Transformer
5x5x4mm = 0.1cc
n/a
Light Sensor
3x3x1mm = 0.009cc
n/a
IGBT
1.5x1.5x0.8mm = 0.0018cc
n/a
Total
~2.2cc
~0.56-1.04cc
Light Machine
Energy Storage
FPC Circuit
Table 3: Flash Reference Design Size Comparison to Equivalent Xenon Flash Solution.
16
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APPLICATION NOTE
Storing Power with Super Capacitors
Conclusion
Until recently, super capacitors have been rarely used in portable systems. Typically they have been limited to back-up
or standby functions that use relatively low currents and offer fairly long charge times. But a new generation of
SmartSwitch™ devices which integrate all the functions needed to limit current, protect the battery or source supply,
continuously charge the capacitor and notify the system when the capacitor is ready for use are changing that scenario. By combining these new smart switches with super capacitors, designers can now create compact solutions
which supply high levels of current for short durations and, in the process, extend battery life or allow the use of
smaller, lighter and less expensive power sources. Super capacitors can operate high current devices such as rf PAs
during transmit for load-leveling and energy storage. The super capacitors are getting smaller with improved temperature and working voltage specifications up to 5.5V and wider temperature range (85°C). Battery life and system
operation is improved to meet portable standards and run time requirements. Future integrated solutions include the
SmartSwitch™ with PMUs to supply portable solid state disk drives that plug into laptop ports.
A current limiting smart switch alleviates the issues with super/super capacitors, inrush control, short circuit protection,
source OV protection and cCurrent flow protection. Using a super capacitor makes it possible to drive very high LED
currents for a super bright WLED flash with two flash LEDs driven at 2A to deliver more light than a K800i xenon strobe.
The super capacitor flash solution is < 2mm thick and can enhance other features in a phone for longer talk time and
better audio while limiting peak current consumed by the battery. A current limiting smart switch with boost converter
eliminates limitations with super capacitors and flash LEDs.
References
Comparison of xenon flash and high current LEDs for photo flash in camera phones
Use of super capacitors to improve performance of GPRS mobile stations
Pierre Mars
CAP-XX Ltd.
9/12 Mars Road
Lane Cove NSW 2066 Australia
http://www.cap-xx.com
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